Petrochemical refiners are facing issues with the utilization of heavy reformate streams. Environmental regulations are limiting the aromatic (C6+) content in gasoline fuel. Gasoline refiners and blenders traditionally use C6+ aromatics, which include the C6-C8 BTEX components (Benzene, Toluene, Ethylbenzene and mixed Xylenes) to improve the octane ratings and the anti-knocking attributes of motor fuels.
Although motor fuel represents a major use of BTEX components, each of the BTEX products has additional commercial uses. For instance, benzene is a widely used precursor for many chemical and solvation processes. Toluene and ethylbenzene are also reactants and precursors in chemical and polymerization processes. The three isomers of xylene (C8H10)—para-xylene (p-xylene), meta-xylene (m-xylene) and ortho-xylene (o-xylene)—are all petrochemical feedstocks of value.
“Mixed xylenes” are a mixture of all three aforementioned xylene isomers. P-xylene is a feedstock for polyester production, a polymer with significant and growing global demand. O-xylene is the feedstock for phthalic anhydride, a precursor for alkyl results and urethane production. M-xylene is used in the production of isophthalic acid and various plasticizers. When mixed xylenes form during catalytic reformation, meta-xylene forms in greater amounts than para-xylene and ortho-xylene because of thermodynamics. However, p-xylene is presently in greatest commercial demand, followed distantly by o-xylene.
Reformate, which is often referred to as the “bottoms” product due to its resulting position at the bottom of a stabilizer during naphtha catalytic reforming, and pyrolysis gasoline (alternatively referred to as “pygas”), a byproduct of ethylene cracking, are the usual sources for these compounds. Extracting the BTEX components from reformate and pygas leaves a heavy reformate, which comprises mainly C9+ alkyl aromatic compounds (pygas may also contain diolefins). Heavy reformate can also form from running hydrogen production processes at a greater severity (i.e., temperature) to produce additional hydrogen alkanes and olefins from the same naphtha feedstock.
The heavy reformate as a chemical stream in and of itself has little direct commercial value, and any ability to “blend away” C9+ alkyl aromatics into gasoline pools is shrinking rapidly.
Toluene is not usually found in heavy reformate as it has been previously recovered or the heavy reformate forms under severe processing conditions such that light aromatics like toluene do not form. Petrochemical refiners that use transalkylation to convert tri-methyl benzenes found in heavy reformate into xylenes often add previously extracted toluene back into the heavy reformate stream. This practice wastes processing energy and unnecessarily increases capital expenditure.